Add a validate method to dir and memory backends

In order to allow all connections to be validated
post-creation add a new method onto the memory
backend (a no-op) and add a new method onto the
dir backend (which checks directory existence) and
raise appropriate errors when validation fails.

Change-Id: Ie476d0c33a970e543ffba21ee8b669ac02258923

taskflow/persistence/backends/impl_dir.py

@@ -95,6 +95,19 @@ class Connection(base.Connection):
         # to restrict the multi-process access does not work inside a process.
         self._lock = backend._lock
 
+    def validate(self):
+        # Verify key paths exist.
+        paths = [
+            self._backend.base_path,
+            self._backend.lock_path,
+            self._flow_path,
+            self._task_path,
+            self._book_path,
+        ]
+        for p in paths:
+            if not os.path.isdir(p):
+                raise RuntimeError("Missing required directory: %s" % (p))
+
     def _read_from(self, filename):
         # This is very similar to the oslo-incubator fileutils module, but
         # tweaked to not depend on a global cache, as well as tweaked to not

taskflow/persistence/backends/impl_memory.py

@@ -78,6 +78,9 @@ class Connection(base.Connection):
     def upgrade(self):
         pass
 
+    def validate(self):
+        pass
+
     @property
     def backend(self):
         return self._backend